Lay pork belly on a work surface, skin side down. Roll pork belly tightly into a log. Tie tightly with kitchen twine to hold log together.

Heat a large dutch oven or thick bottom pot over high heat; add pork belly log and sear evenly on all sides until crispy golden brown.

In a large mixing bowl, combine sugar, soy sauce, mirin, sake, and 3 cups water. Stir until sugar is completely dissolved.

Pour the sauce mixture into the pot along with ginger, garlic, and leek/green onions. Stir to ensure even braising. Bring to a boil without cover.

Once boiling, cover, reduce heat to medium-high, and simmer for 1 hour and 30 minutes. Then reduce heat to medium-low and simmer for an additional 30 minutes, rolling the pork in sauce every 20 minutes.

If the sauce has not reduced enough, remove the lid and increase heat to high to thicken the sauce.

Remove from heat and let cool for 30 minutes. Once cooled, place the pork log into a plastic bag with 1 cup of sauce, close tightly, and refrigerate overnight.

Remove twine from the portion you will use. Slice pork chashu to desired thickness and sear by pan frying on a dry pan over high heat, grilling, or using a torch. Serve on top of ramen, warm rice, or in steamed bao.